Phase Evolution and Identification in Beta-Ti-Nb Alloys

The push for increased functionality and high temperature stability of future structural materials prompts increasing attention towards body centered cubic (bcc) alloys, primarily refractory- or Ti-based. In light of the increased demand for sustainable alloying systems with simplified base element constitution, the present study focuses on investigations in the binary Ti-Nb system. As a sub-class of the bcc beta-Ti alloys, the Ti-Nb system is characterized by the presence of a metastable bcc structure at room temperature, and allows for a combination of the high temperature refractory stability of Nb with the remarkable ductility and functionality of Ti-based alloys.<br/>Of interest is specifically the equimolar Ti-50Nb composition, which falls inside a predicted chemical spinodal. This spinodal can be tuned by the addition of oxygen, prompting a decomposition into Ti-rich and Nb-rich domains that can act as precursors for further phase transformations.<br/>The present study investigated two aspects of these phase transformations through targeted addition of up to 1.4at%O in the Ti-50Nb system:<br/>1) The thermally induced evolution of a hcp-bcc network during a controlled heat treatment within the chemical spinodal, as a strengthening mechanism in Ti-50Nb. This was facilitated by heat treatments of dehydrided powder alloys in a controlled O atmosphere.<br/>2) The stress induced (martensitic) evolution of phases along the bcc to hcp transformation pathway prompted by subcooling into a metastable state from the single-phase bcc region, to promote low modulus pseudoelasticity [1]. This was facilitated by heat treatments of bulk samples prepared by arc melting with the addition of controlled amounts of TiO₂.<br/>Significant changes in mechanical properties were tracked using a combination of thin foil tension and micropillar compression, and linked to hcp or martensitic phase evolution by detailed phase analysis on the basis of transmission electron microscopy (TEM) and micro-Laue diffraction. The results are presented in the framework of wider refractory multi-principal element alloy (RMPEA) performance, the role of impurity elements in bcc alloys, and the application of advanced diffraction-based characterization for the tracking of phase evolutions in these systems.